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A loaf of bread or tickets to a football match do not cost the same as they did in 1891. If you hold on to a PS20 note for forty years it will not buy you as much as it did then. Inflation is calculated in various ways, but the measures you are most likely to hear about are the Consumer Prices Index (CPI) and the Retail Prices Index (RPI). These are compiled by surveying the rising or falling cost of a basket of different goods and services. RPI and CPI record different things - RPI includes housing costs, for example - and they are calculated differently, so the rates are not the same. Usually CPI is lower and tends to be what is considered the national statistic of inflation.